Thermostat

ABSTRACT

A thermostat for an electric heater containing a thermal relay and a temperature-sensing unit in the same assembly. The thermal relay is mounted above the temperature sensing unit so that the convection air currents resulting from the heat generated by the thermal relay induce room-temperature air to flow past the temperature-sensing unit and this effect speeds up the response of the temperature sensor to the ambient temperature of the room. Heating of the relay contacts due to the relatively heavy heater current helps to induce the convection air currents and does not cause any appreciable &#39;&#39;&#39;&#39;droop.&#39;&#39;&#39;&#39; An anticipator resistor and the ambient temperature sensing bimetal are mounted together on a heat-exchange member. This promotes a short cycling time, as required for effective room heating, taking into account the response times of the thermal relay. The operating bimetal of the thermal relay is mounted on a compensating bimetal element to insure operation of the thermal relay in response to the temperature-sensing unit even under extreme weather conditions. A manual control is provided to open the relay contacts and to operate an additional switch for breaking the room-heater curcuit at both sides of the line.

United States Patent Norden [1: 3, 665,360 [451 May 23,1972

1541 TEER'MOSTAT [72] Inventor: Alexander R. Norden, 350 Central Park West, New York, N.Y. 10025 22 Filed: Mal-.11, 1970 211 Appl.No.: 18,492

2,611,056 9/1952 Jackel ..337/99 I FOREIGN PATENTS OR APPLICATIONS 1,524,411 5/l968 France ..337/l00 Primary Examiner-Harold Broome Assistant Examiner-F. E. Bell Attorney-George Gottlieb, Michael I. Raekman and James Reisman v as) l ABSTRACT A thermostat for an electric heater containing a thermal relay and a temperature-sensing unit in the same assembly. The thermal relay is mounted above the temperature sensing unit so that the convection air currents resulting from the heat generated by the thermal relay induce room-temperature air to flow past the temperature-sensing unit and this efiect speeds up the response of the temperature sensor to the ambient temperature of the room. Heating of the relay contacts due to the relatively heavy heater current helps to induce the convection air currents and does not cause any appreciable droop." An anticipator resistor and the ambient temperature sensing bimetal are mounted together on a heat-exchange member. This promotes a short cycling time, as required for effective room heating, taking into account the response times of the thermal relay. The operating bimetal of the thermal .relay is mounted on a compensating bimetal element to insure 29 Clains, 1o DrawlngFigures 2 Sheets-Sheet 1 Patented May 23, 1972 d M s 5 I m 1.. n fT N 3 L m M m i a X. a. w 2 m I 7 4 m M A a. 3 2% Z i F 6 f l L F 7 M v 5 T M an 4 6 my 3 F a W 4 v 5 aw 22/ m M i 2 mwzflw 2 z a 6 fl w w w w z I BL 3 \5 7 NW ill- I? n 1nd W O 1 W H z J TIL. 52 t 7 fl L, fla p 2 f w 7, 3T M fl This invention relates to thermostats, and more particularly to a thermostat containing load-break switching contacts for connecting a power line directly to an electric heater.

A ccnventionalthermostat includes a temperature-sensing unit for determining when heating of the room in which the thermostat is placed should begin. In the case of a thermostat for controlling an electric heater, the temperature-sensing unit (which typically includes a bimetal element) causes contacts to close which in turn connect "the power line to the heater. The contacts can be included in the thermostat itself (in which case both the power line and the heater line are connected to terminals on the thermostat) or they can be situated somewhere .else with the temperature-sensing unit simply transmitting a signal to a relay or some other mechanism which causes the contacts to close and connect the power line to the heater. The main problem with incorporating the loadbreak contacts in the thermostat itself is that so much heat can be generated in the thermostat as a result of the large (as muchas 25 amperes) current flowing from the power line to the heater that the temperature-sensing unit no longer responds to the ambient temperature exclusively; to a great extent'it operates in accordance with the heat generated in the thermostat itself. This heating is a primary cause of droop, a term used to'signify the difference between the temperature at which the thermostat actually regulates and the desired or set temperature.

It is a general object of my invention to provide a thermostat including both a temperature-sensing unit and load-break contacts in which the heat generated by current flowing through the load-break contacts does not have any appreciable effect with operation of the temperature-sensing unit.

It is well known that for minimum fluctuations of room temperature, the room heater should be turned on and off quite often. in cold weather, the on time of each cycle increases relative to the off time, but' the more frequent the overall cycling the less the deviation from the desired temperature.

It is another object of my invention to provide a temperature-sensing unitwhich accurately responds to the room temperature, prevents excessive deviations from the desired temperature,and cycleson and off at a relatively high rate (6-9 cycles per hour in the illustrative embodiment of the invention).

The illustrative embodiment of the invention utilizes a thermal relay, including an operating bimetal and a resistance heating element on that bimetal. When the temperaturesensing unit determines that the room heater should be turned on, asmall current flows through the heating element which in turn heats the bimetal element. The bimetal element bends and causes the load-break contacts to close so that room heatercurrent can flow. Extremes of ambient room temperature, and heating caused by the relay itself, could interfere with the intended response of the relay contacts to the control current supplied to the bimetal heating element. The contacts might fail to close at very low temperatures, and the contacts might remain closed due to high ambient temperatures in the relay enclosure. All of this is well known. Bimetal-operated thermal relays are commonly provided with ambient temperature compensation.

it is another object of my invention to provide a bimetal element thermal relay for closing the relay contacts, in which ambient temperature compensation is provided in a single, effective and economical manner, and, more particularly, a bimetal element thermal relay that has changing degrees of compensation at different relay ambient temperatures. Such a relay is particularly well adapted to respond to the ambient room temperature sensing unit.

Another object of my invention resides in providing a twopole thermostat having relay-operated contacts controlling one side of a heater circuit, and an additional switch independent of the relay for breaking the return line of the heater circuit, with a common manual operator for both; and more particularly a manual control that sequences the contact opening so that only the relay contacts are needed for load-break duty.

The additional switch can then be of simple construction since it need not be designed to withstand arcing as the heater current is turned on and off.

In the illustrative embodiment of my invention, a thermal relay is disposed near the top of the thermostat. The temperature-sensing unit is disposed at the bottom of the thermostat. The thermal relay has a heating element on a relay operating bimetal, and the heating element is connected in series with the temperature-sensing unit across the power line. When the temperature-sensing unit determines that the room heater should be turned on, current flows through the heating element and the relay contacts close. Heat is generated in the upper portion of the thermostat-both by the heating element of the thermal relay and by the room heater current which flows through the thermal relay contacts. Care is taken to prevent this heat from affecting the temperature-sensing unit by mounting the temperature-sensing unit at the bottom of the thermostat and by introducing thermal isolation between the temperature sensing unit and the upper part of the overall assembly. As a feature of special significance, the increased temperature ofthe upper part of the thermostat heats the air in its immediate vicinity and causes a convection air current to flow up along the thermostat. The current of airat room temperature is especially effective in causing the temperature-sensing unit to respond to the ambient temperature, much more effectively than if the temperature sensing unit were exposed to static room air. In contrast to usual thermostats where contact heating causes a droop" problem, here the contact heating is turned into an advantage since it tends to increase the convection current past the temperature sensing unit. The temperature-sensing unit is thusable to respond to the room ambient temperature without being affected appreciably by the total relay heat generated in the upper part of the thermostat.

, In the preferred construction, the temperature-sensing unit itself is a plug-in unit containing a bimetal sensing element. This plug-in construction contributes to the thermal isolation of the temperature sensing unit from the heat developed in the thermal relay. The temperature-sensing unit includes a heatexchange element in good heat-transfer relationship with the temperature sensing element which, in turn, is in good heatexchange relationship with an anticipator resistor mounted directly in a bore of the heat-exchange member. This construction contributes greatly to the increased cycling rate in a stable manner for effective room heating, compatible with the thermal relay response time. A plurality of slots expose the heat-exchange element to the atmosphere. The contacts operated by the temperature sensing bimetal are completely enclosed for minimizing exposure tov insects, dust, etc. The temperature-sensing unit has a thin-walled enclosure of electrical insulation of small thermal mass. This provides protection against contact with energized parts. Despite the protective enclosure of the sensing element, the use of the heatexchange element allows the sensing element to closely follow the ambient temperature.

In the thermal relay of the thermostat, the operating bimetal is mounted on a compensating bimetal element. With an increase in temperature, the compensating bimetal bends in the opposite direction from the operating bimetal element. At cold temperatures there is full compensation; consequently, current flowing through the heating coil is always sufficient to close the thermal relay contacts no matter how cold it is in the room. Compensation is also provided at higher temperatures so that the thermal relay contacts open when the energization of the heating element is interrupted. However, full compensation at higher temperatures theoretically tends to neutralize the contact-closing action of the energized heating element, since the compensation is heated by conduction from the operating bimetal and acts in the direction to open the contacts. This anomalous tendency is reduced by blocking part of the length of the compensating bimetal element at higher temperatures. The reduced compensationinsures proper operation of the relay contacts while the heating element is energized.

through line 6-6 of FIG.

The manual operator includes a cam for opening the thermal relay contacts at one side of the line even if the heating element is energized. The manual operator also operates a commutator switch to open the other side of the line of the room heater circuit. Moreover, the cam is situated such that when the manual operator is turned on" the commutator switch closes prior to closing-of the thermal relay contacts, and when the manual operator is turned off" the thermal relay contacts open prior to the opening of the commutator switch. Thus it is always the heavy-duty thennal relay contacts which make or break the circuit, and the commutator switch need not be designed to withstand arcing.

Further objects, features, and advantages of my invention will become apparent upon consideration of the following detailed description in conjunction with the drawing, in which:

. FIG. 1 is a front view of the illustrative embodiment of my invention; I

FIG. 2 is a rear-view of the thermostat (with cover plates 63 and 64 removed);

FIG. 3 is a sectional view taken through the line 3-3 of FIG. 2;

FIG. 4 is a-sectional view taken through the line 4-4 of FIG. 2;

FIG. 5 is a plan view of manual operator l3 and control circuit switch contact 57;

FIG. 6 is a sectional view of manual operator 13 taken FIG. 7' is a sectional view taken through line 7-7 of FIG. 1;

FIG. 8 depicts the mounting of the thermal relay on compensating bimetal element 42;

FIG. 9 is a schematic diagram illustrating the circuit connections of the thermostat in the off position; and

FIG. 10 is'a side view showing the mounting of the thermostat to a wall box and the convection air currents which flow past the thermostat.

In FIG. 1, the main part 10 of the illustrated thermostat sup ports a plug-in temperature sensing unit 11. There are two holes 12 for mounting screws that fasten the thermostat to a standard electrical box B (FIG; 10) that is set in wall W. Unit 11 is wholly below box B, so that unit 11 is not exposed to the heat in box 13. On-off manual operator 13 is pivoted for manual operation between the on position shown in FIGS. 1, 3 and 4 and the opposite off position shown in dotted lines in FIG. 4. Knob 14 on plug-in unit 11 is adjustable for establishing the set point of the thermostat.

TEMPERATURE-SENSING UNIT 11 Plug-in unit 11 is here secured to main part 10 by a single screw 15(FlGS. 1 and 7). Unit 11 includes a split case of molded insulation, including parts 16 and 18. One casing part 16 has a projection 16a that enters a cavity in molded body 17 of main part 10. Ribs 17a extending integrally from body 17 (FIGS. 2 and 7) limit the area of contact between body 17 and easing part 16, for restricting heat conduction from main part 10 to the plug-in temperature sensing unit 11.

Main casing part 16 and cover 18 are suitably united, as by screws or drive pins (not shown). The cavity formed by the casing parts contains an elongated heat-exchange member 19. This member is a good conductor of heat both because it is made of a high-conductivity metal such as aluminum and because it has substantial thickness. Ends 19a and 19b are in complementary recesses in casing l6, 18, so as to locate member 19 firmly in position. There is a series of passages 20 in casing parts 16 and 18 along the bottom wall of the casing. These passages admit air at room temperature to heatexchange member 19. Each passage 20 is narrow, and it has a sharp bond, for preventing a straight metal part when inserted into a passage 20 from touching member 19. A child might poke at passages 20 with a pin, for example. As will be clear from FIG. 9, explained below, member 19 is held at the potential of bimetal 21, which may be at 1 10 volts AC above ground or higher. Therefore, if member 19 were exposed to direct contact from outside the casing, there would be an electrical shock hazard. Member 19 has projections 19c extending into the bent passages 20, but even projections are relatively inaccessible.

A bimetal strip 21 has one end anchored in a compressed slot in member 19 at the left in FIG. 2. Screw 22 close to the anchoring point of bimetal 21 adjusts the position of contact 23 carried by the bimetal. A companion contact 24 and a small permanent magnet 25 are carriedby a metal spring arm 26. An eccentric pin 27 integral with knob 14 bears against spring arm 26, for adjusting the set-point of the thermostat. Pivot enlargement 27a (FIG. 7) is blocked by spring arm 26, so that knob 14 and eccentric pin 27 as a unit are retained in the assembly as shown.

Bimetal 21 is of ferromagnetic material. Therefore, when contacts 23 and 24 come near each other, magnet 25 causes the contacts to close suddenly and firmly. When bimetal 21 cools a little, sufficient spring bias develops to overcome magnet 25, and then the contacts part abruptly. The difference in temperature of the bimetal between opening of the contacts and closing of the contacts is readily made quite small, in accordance with usual practice in magnet-assisted thermostat contacts. I I

Member 19 and casing parts 16 and 18 of temperaturesensing unit 11 provide a completely enclosed and protected space for the bimetal and the contacts. A complete enclosure is important for minimizing exposure of the contacts to deposits of dust andfilm from the air, for guarding against tampering, and for preventing insects from entering the space, all of which could interfere with the proper operation of the contacts.

Despite the protective enclosure of bimetal 21, several factors promote effective response of this bimetal to the temperature of the air outside casing 16, 18. First, the casing 16, 18 has thin side walls and a large area so that contained bimetal 21 tends to follow closely the outside air temperature. Additionally, heat-exchange member 19 helps bimetal 21 in following the outside air temperature, since ('1 member 19 is greatly elongated, (2) member 19 is in good heat-transfer relation with the anchored end of the temperature-sensing bimetal, (3) heat-exchange member 19 is in good heat-transfer relation to casing 16, 18, especially the bottom wall of the casing, and (4) parts of memberl9-are exposed directly to the outside air. Member 19 has a high ratio of surface to thermal mass. Because of these factors, member 19, has a tendency to follow closely the ambient temperature outside casing 16, 18 and member 19 correspondingly induces bimetal 21 to follow the ambient air temperature. In addition, the thermostat creates a convection air current, as explained below and shown in FIG.

.10. This air current induces unit 11 as a whole to follow closely the outside air temperature.

ANTICIPATOR 28 IN UNIT 11 At end 19a of member 19 there is a cavity containing a small low-wattage resistor 28, called an anticipator." In the thermostat as shown, this resistor is a commercial one-tenthwatt resistor resembling a ceramic rod with wire terminals at its ends. The resistor is contained in a bore in end 19a of heatexchange member 19 in good heat-transfer relationship so that virtually all of the heat developed in the resistor is transferred to end 19a of member 19.

One terminal of resistor 28 is connected to a metal strip 29 received in a groove in casing projection 16a. The end of this strip forms one plug-in contact 30 of unit 11. Spring arm 26 extends integrally along another groove in projection 16a to form another plug-in contact 31.

Anticipator resistor 28 carries only a small amount of current when contacts 23, 24 are closed, and its resistance is small. Consequently resistor 28 develops a very small amount of heat when it is energized. Virtually all of the heat developed in resistor 28 is transferred to end 19a of heat-exchange member 19. The portion of bimetal 21 nearest end 19a is most sensitive, in that a small temperature rise at that region causes a substantial movement of contact 23, whereas the same small temperature change near contact 23 causes very little movement of the contact. So long as contacts 23, 24 stay closed, anticipator 28 continues'to supply heat to end 19a so that its temperature reaches the preset control temperature of the thermostat before room ambient temperature reaches it. The

rate of rise is limited by the effect of member 19 in conducting heat away from end 19a. Nevertheless, the temperature of end 19a and that of the sensitive end of bimetal 21 rise above the ambient temperature. Hence, the contacts 23, 24 separate somewhat before the room ambient temperature reaches the preset control temperature. As explained below, this interrupts the supply of heat to the room, and it interrupts the heating current to anticipator 28.

After contacts 23, 24 part, the temperature of end 19a is quickly reduced by conduction along heat-exchange member 19. The heat previously supplied to member 19 by anticipator 28 is quickly dissipated because of the relatively large surface area ofmember 19 (compared to its thermal mass), the large area and thin walls of casing. 16, 18,'and the convection air current. Accordingly, member 19 and bimetal 21 rapidly approach the temperature of the air around unit 11 after anticipator 28 is de-energized. Thus, if the room temperature is at thatpoint not equal to the preset control temperature contacts 23 and 24 will re-close. As an over-all result, there is a desirably short cycle-time of contacts 23, 24 opening and closing under the influence of anticipator 28 and the heat dissipation means. The rapid cycling that can be obtained with the described features is important in limiting the swings of room temperature above and below the set point. In each relatively short operating cycle, the percentage of on" time increases as the heating requirements increase. In a practical form of thermostat made as shown, the cycling rate was a very satisfactory 6-9 cycles per hour. This represents a big improvement over usual commercial line-voltage thermostats which, in some cases, are reported to have a cycling rate of between I and 2 cycles per hour, RELAY UNIT Mainpart of the thermostat has a body 17 of molded insulation. Near the top' of body 17 there is a rear cavity 33 (FIG. 4) containing a thermal relay 34. This relay includes a moving contact 35 that cooperates with another contact 36. A metal member having flexibleside arms 37 (FIG. 2) joined by a bridging part 38 carries moving contact 35. An operating bimetal 39 is mounted between 'arms 37 and is arranged so that its active right-hand end (as viewed in FIG. 4) rises when the bimetal is heated. A pair of C-shaped spring members 40 is confined between the active end of bimetal 39 and bridging part 38 that carries the moving contact 35.

With operating bimetal 39 and the moving contact 35 in the positions shown in FIG. 4, springs 40 act to bias the moving contact upward, thereby holding the thermal relay contacts open. When the active end of bimetal 39 rises, a dead center point is reached. Further upward deflection of the active end of bimetal 39 reverses the action of springs 40, causing snapclosing of contacts 35, 36. The reverse motion of the active end of bimetal 39 due to cooling of the bimetal causes snapopening of the contacts. Energization of heater 41 on bimetal 39 causes closing of the contacts. Heater 41 is usually made of resistance wire wound around the bimetal (see FIG. 8) and is electrically insulated from the bimetal, the whole assembly being wrapped in insulation. When bimetal 39 is at normal room temperature, contacts 35, 36 are parted. That is one stable condition of the thermal relay.

Y AMBIENT-TEMPERATURE COMPENSATION FOR THERMAL RELAY 34 At its left-hand end as shown in FIGS. 4 and 8, operating bimetal .39 is welded or otherwise suitably united to the movain FIG. 4, and compensating bimetal 42 warps in the opposite direction. Full compensation occurs when the combined effects of an ambient temperature rise or decline on both bimetals results in the right-hand or active end of bimetal 39 remaining unmoved. It is known that I00 percent compensation is realized in the case of one bimetal supporting another when the lengthsof the supported bimetal to the supporting bimetal have very nearly the ratio 2.4 to 1.0.

Thermal relay 34 operates when the active end of bimetal 39 causes overcentering of springs 40. In a small and efficient thermal relay, the active end of bimetal 39 may shift less than 0.05 inch after the bimetal has been heated for a reasonable interval to cause overcentering of springs 40. Conversely, when heater 41 is de-energized and bimetal 39 is allowed to cool, the active end of bimetal 39 may shift'reversely less than 0.05 inch, reversely overcentering springs 40 and opening the contacts.

One design of such a relay without ambient temperature compensation was found non-responsive to heater 41 after a reasonable period of heater energization when the room space to be heated was at a temperature of -30 F. Furthermore, the uncompensated relay would not reliably open contacts 35, 36 in a F. room ambient after heater 41 was deenergized but with the relay cavity 33 heated by 25A current passing through contacts 35 and 36. These factors indicate that ambient compensation is desirable.

On the other hand, when bimetal 39 is heated by heater 41 in an ambient compensated relay, the temperature of compensating bimetal 42 also rises due to direct conduction of heat from operating bimetal 39. Thus the tea] deflection of the ambient compensating bimetal 42 is the sum of effects of ambient temperature and conducted heat. While, therefore, full compensation is very desirable at low ambient temperatures to ensure positive overcentering action to an on" position, it is equally desirable to limit the deflection of the compensating bimetal 42 at normal energized-heater (41)" temperatures so that the sum of the ambient-temperature and conductedheat effects produces a deflection approximately equal to that which would have been obtained with full compensation in the absence of the conducted-heat effect; this is because the sum of the two effects on a fully active compensating bimetal would drive the active end of bimetal 39 down farther than desired, tending to open contacts 35 and 36.

The reduced activity of the compensating bimetal is achieved by mounting part 42a against a flat surface of bracket 43, and fastening the free end of part 42a as by rivet 44. At normal room temperatures, bimetals 39 and 42 assume the positions shown in FIG. 8. At lower temperatures, since both parts 42a, 42b can bend (part 42a bends away from bracket 43) there is full compensation. But at higher temperatures, part 42a cannot bend (clockwise relative to rivet 44 in FIG. 8) because it is blocked by bracket 43. Bimetal part 42b remains operative to provide only a limited degree of compensation, as desired.

RELAY CONNECTIONS Arms 37 carrying the movable contact 35 extend integrally from bridging part45 that is pivoted on an edge of part 42b and connected by a metal strip to one line terminal 46 of the thermostat. Contact 36 is joined to a metal strip 47 that is fixed to insulation member 17. Strip 47 extends integrally to load terminal 48 of the thermostat. One terminal of heater 41 is joined to bimetal 39 and connected via bimetal 42 and bridging part 45 to line terminal 46. A wire 49 extends from the opposite terminal of heater 41 to a receptacle contact 50 formed of a strip of resilient metal with a reverse-bent part that presses against plug-in contact 30.

A second line terminal 51 carries a resilient two-fingered wiper contact 52 (FIGS. 3 and 9) that bears against commutator contact 54 of manual operator 13. A second resilient twofmgered wiper contact 55 bears against commutator contact 54 and is connected to load terminal 56 of the thermostat.

When manual operator 13 is turned to its off position, shown in dotted lines in FIGA, commutator contact 54 parts from contact 52. This interrupts the connection of load terminal 56 to line terminal 51 as shown in FIG. 9.

Commutator contact 54 has a part 54a engaged by a second resilient wiper contact 57 (FIGS. 2, and 9) that extends as part of a metal strip to receptacle contact 58 engaged by plugin contact 31. In the off position of manual operator l3, commutator contact 540 separates from wiper contact 57 thus isolating contact 57 from both line terminal 51 and load terminal 56. Parts 52, 54, 55, 54a and 57 constitute a doublepole single-throw commutator switch.

A leaf-spring member 59 (FIG. 4) has one end fixed to bracket 43. Part of member 59 is a cam-follower 59a that bears against manual operator 13. A projection 59b underlies a hook portion 60 of bridging part 38 carrying the moving contact 35. When bar 13 is moved manually to its dotted-line position (FIG. 4), cam-follower 59a enters cam cavity 61 and projection 59b engages hook portion 60 to hold contact 35 spaced from contact 36. In the position of manual operator 13 shown in solid lines in FIG. 4, cam follower 59a enters a shallow cam cavity 62, and acts as a detent to hold manual operator 13 in that position.

The circuit connections of the thennostat are shown in FIG. 9, in the off position. Contact 35 is shown parted from contact 36 in FIG. 9 as a resultof cam follower 59a having entered cam cavity 61. Commutator contact 54 is parted from wiper contact 52. Therefore both load terminals 48 and 56 are disconnected from their respective line terminals 46 and 51. Further, a circuit extends from line terminal 46 through heater 41, anticipator 28, and thermostat contacts 23, 24 (when closed) to wiper contact 57. In the off position shown, contact 57 is parted from commutator contact 54a to interrupt this circuit.

Contacts 35, 36 are proportioned and of a material capable of many operations in interrupting the load current to the room heater, 25 amperes for example. Commutator 54 and wiper contact 52 are proportioned to carry the load current, but they are not designed for making or breaking a circuit carrying load current. For this reason, cavity 61 and cam follower 59a are related to commutator 54 so that contacts 35, 36 are allowed to close only after commutator 54 is engaged by wiper 52 and, conversely, commutator 54 breaks contact with wiper 52 only after cam follower 59a drives contacts 35, 36 apart.

The electrical terminals 46, 48, 51 and 56 are freely exposed for connection to external wiring in box B. The thermal relay 34 is covered byv a plate of insulation 63 (FIG. Another insulating plate 64 (FIG. 7) covers the area of main part 17 of insulation opposite part 16a of the plug-in unit; plate 64 covers the areas at both sides of a raised part 65 (FIG. 2) at the lower mounting-screw hole 12.

OPERATION In normal operation of the thermostat, manual operator 13 is in its on" position. Projection 59b is out of range of hook 60 as shown in solid lines in FIG. 4. Commutator 54, 54a connects line terminal 51 and contact 52 to contacts 55 and 57.

When the temperature of bimetal 21 drops below the set point, contacts 23, 24 close. Current flows in the circuit from line terminal 46, heater 41, contacts 50, 30, anticipator 28, bimetal 21, contacts 23, 24, contacts 31, 58, and contacts 57, 54a, 52 to the opposite line terminal 51. Energizing heater 41 in this way causes contacts 35, 36 to close. In a sample thermostat made as shown, heater 41 draws about 3 watts, and bimetal 38 is heated enough to close contacts 35, 36 in typically 30 to 40 seconds. Operation of the room heater starts when contacts 35, 36 close.

Since anticipator 28 is in series with heater 41, anticipator 28 is also energized when contacts 23, 24 close. In the sample thermostat mentioned above, the resistance of heater 41 was 4,700 ohms, for a l-volt circuit, and the resistance of resistor 28 was 62 ohms. Accordingly, anticipator 28 developed about 0.04 watt in the sample thermostat. This heat gradually raises the temperature of heat-exchange member 19 (FIG. 2). The heat is transferred efficiently to end 194 where bimetal 21 is most sensitive, but the heat is also conducted efficiently along member 19. The effect of anticipator 28 when energized, and of member 19 when heated, is to expose bimetal 21 to a gradually developing temperature rise above room ambient. In the sample thermostat this temperature rise was l.5 F. above ambient. Under usual conditions, anticipator 28 causes contacts 23, 24 to open before room temperature reaches the preset control temperature. This is desired to prevent the thermal energy stored in the room heaters from making the room temperature overshoot" the desired temperature. Operating bimetal 39 of the thermal relay cools rapidly when its heater 41 is de-energized by opening of contacts 23, 24, and then contacts 35, 36 open to interrupt the current supply to the room heater. Heat exchange member 19 and casing 16, 18 rapidly dissipate the heat developed by anticipator 28. Assuming that the actual ambient temperature of the room is below the set point established by adjusting knob 14, contacts 23, 24 close again and energize the room heaters.

The contacts 23, 24 close, open and close repeatedly in relatively short cycles. The time of contact closure is a large part of the cycle and the open time of the contacts is short in very cold weather; and the reverse is true (short closed" time and long open time) in weather that requires only little heating in the room. The cycling time differs somewhat under these different conditions. A frequency of 6 to 9 cycles per hour was realized in the sample thermostat mentioned above. The room heating tends to be relatively uniform when such rapid cycling occurs since both the off" times and the "on" times of the room heater are relatively short. Only limited excursions of room temperature occur. The limited amount of anticipator heat that is needed and the rapid dissipation of the anticipator heat by heat-exchanger 19 and casing 16, 18 after the anticipator is de-energized enable the bimetal 21 to respond to the actual room temperature, free of anticipator heat, to reclose contacts 23, 24 when necessary. This is significant in that, if bimetal 21 were still exposed to long-retained anticipator heat, room temperature would drop substantially below the preset control temperature before contacts 23 and 24 closed. That effect is a kind of droop attributable to the stored anticipator heat, and is present in usual line-voltage thermostats.

FIG. 10 illustrates a notable operating characteristic of the thermostat. Heater 41 of the relay develops a significant amount of heat and relay contac s 35, 36 develop some additional heat. In the sample mentioned above, heater 41 developed about 3 watts and contacts 35, 36 developed about l.5 watts with a load current of 25 amperes, the continuouscurrent rating of the contacts 35, 36. This heat raises the surface temperature of molded part 17 over a large area in front of relay 34, significantly above room ambient temperature when the on time of the relay is long and the ofl time is short. This has the effect of creating a prominent convection current of air, as indicated by the arrows. Even when the controlled current of the room heater is small and the contact heating is reduced, the heat from heater 41 develops efi'ective convection air currents. Temperature sensing unit 11 is exposed to this turbulent convection air current. This has the effect of avoiding a dead air space about the temperaturesensing unit 1 1 which is therefore more effective in sensing the room ambient temperature. Further, the convection air current tends to carry away any heat from temperature sensor 1 1 that travels by conduction from relay 34 downward in molded body 17. In the sample thermostat mentioned above, a temperature rise of only'3.4 F. occurred at temperature-sensing unit 11 due to heat conduction with 22A room heater current and percent on time, when molded member 17 was made of a glass-filled epoxy having a heat conductivity of 4.3 BTU's per hour per square-foot per F. per inch. This is comparable to phenolic molding material. In the preferred construction, the heat conductivity is reduced further by using other molding materials, such modified polyphenylene oxide, commercially available as Noryl SE-l made by General Electric. This material has a heat conductivity factor of 1.5

and-still meets the temperature and electrical requirements of the thermostat shown. The temperature rise at unit 11 resulting. from heat conduction from relay 34 is at its maximum when heater 41 is energized continuously or nearly continuously and contacts 35, 36carry 25A. There is less heat conduction when there are short on" timesand long off times in the on-off cycles of thermostat operation since there is a lower average amount of relay heat and consequently a reduced average temperature rise in molded member 17 at relay cavity 33. Under these conditions there is less need for the convection air current induced by the relay heat since the relay develops a lower amount of heat as averaged by main molded part 17.

The limited heat conduction in body 17 is restricted from reaching unit 11 by the limited areas of contact at ribs 17a and at the electrical plug-in contacts 30, 50 and 31, 58. And the temperature rise of unit 11 due to heat conduction from relay 34 is additionally reduced by the convection air current. Similarly, the heat developed in unit 11 by anticipator 28 is quickly dissipated by the convection air current when the anticipator is de-energized, since the insulating body 17 with its relatively high thermal mass retains its elevated temperature, and hence creates convention currents for substantial time after relay de-energization.

' The amount of droop of the illustrated thermostat is representedby the limited amount of heat conduction from relay 34 to temperature sensor 1 1 andthe slight droop caused by the anticipator. Notably, the amount of droop is almost unrelated to the amount of load current and contrasts sharply with other line-voltage thermostats generally on the market today, where. a thermostat having only" a 14 droop is considered a premium thermostat and where standard commercial units have droops varying from 25 to 70 F. The present thermostat is of low-cost construction, so as to be salable as a competitive"thermostat. Unlike competitive thermostats, the present thermostat features both low droop and short cycling-time characteristics.

Manual operator 13 serves as an on-ofi" switch. In moving to an elf position cam cavity 61 causes contacts 35, 36 to part (if they should be closed) so as to interrupt load current, and in continuation of the movement contact 54 parts from contact 52. Contacts 35, 36 are well suited to load-breaking duty. Opening of contacts 52 and 55 provides assurance of having both load wires disconnected from the line. Inthe ofFposition of manual operator 13, contacts 54a, 57 are parted, so that'the heater circuit of the thermal relay is also broken. Contacts 54, 52 are inexpensive, and are designed to open after contacts 35, 36 have already opened the circuit, in order: to avoid pitting of contacts 54, 52 due to arcing, and thus to avoid a potentially heat-generating connection in a location relatively close to the sensing unit 11. The described thermostat thus incorporates a safe and economical ofi switch.

The thermal relay has anoperating bimetal and a compensating bimetal, arranged to operate load-break contacts with snap action in both making and interrupting the circuit to the room heaters. The deflection of the operating bimetal in response to its heater is maintained uniform over a wide range of ambient temperatures that are probable and even usual in various environments because of the compensating bimetal. The compensating bimetal 42 has part of its length blocked when above nonnal room temperatures are reached, temperatures caused by the heat reachingthe compensating bimetal due to heating of bimetal 39 by heater 41. This construction provides for ambient temperature compensation under conditions when it is needed, but prevents the ambient temperature compensation from interfering with the intended operation of the operating bimetal 39.

Although the invention has been described with reference to the particular embodiment, it is to be understood that this embodiment is merely illustrative of the application of the principles of the invention. Numerous modifications may be made therein and other arrangements may be devised without departing from the spirit and scope of the invention.

What is claimed is:

1. A thermostat for monitoring the ambient temperature in a region to be heated and for controlling the connection of an electric supply line to an electric heater for said region, said thermostat including a body, supply terminal means for connection to an electric supply line, and heater terminal means for connection to an electric heater, a thermal relay carried by said body and having relay contacts operative to complete and to interrupt a current path between said supply terminal means and said heater terminal means, the heat of said thermal relay inducing the flow of convection air currents, and ambient temperature-sensing means difi'erently responsive to the ambient temperature above and below a set point controlling the energization of said thermal relay, said ambient temperature-sensing means being carried by said body below said thermal relay in said convection air current.

2. A thermostat in accordance with claim 1 further including a pair of holes on said body for enabling the mounting of said thermostat to a wall electrical box such that when it is mounted to said wall electrical box said ambient temperaturesensing means is situated below the bottom of said box.

3. A thermostat in accordance with claim 1 wherein said ambient temperature-sensing means includes control contact means for making and breaking an energizing circuit to said thermal relay, an ambient temperature-sensing bimetal strip for controlling the opening and closing of said control contact means above and below a set point, and a closed casing of insulation containing said bimetal strip and said control contact means for guarding the contents of said casing against access and thereby minimizing electrical shock hazard.

4. A thermostat in accordance with claim 3 wherein said casing includes a wall having a plurality of slots providing air passages to said ambient temperature-sensing means for promoting rapid response of said bimetal strip to the temperature of the convection air currents, each of said slots including a sharp bend for inhibiting access to the interior of the casing for minimizing electric shock hazard.

5. A thermostat in accordance with claim 1 wherein said ambient temperature-sensing means is a separate unit contained in a casing secured to said body, said body and said casing having juxtaposed mechanical and electrical connecting portions and said connecting portions having formations for restricting the physical heat conduction path from said body to said separate unit.

. 6. A thermostat in accordance with claim 1 wherein said ambient temperature-sensing means includes a casing of electrical insulation, control contact means in said casing for selectively energizing said thermal relay, metallic heatexchange means disposed in said casing, a bimetal strip secured at one end thereof to said metallic heat-exchange means and arranged to operate said control contact means, and an anticipator resistor electrically connected to said control contact means and mounted in intimate heat-exchange relationship with said metallic heat-exchange means.

7. A thermostat in accordance with claim 6 wherein said anticipator resistor is mounted adjacent to said secured end of said bimetal strip.

8. A thermostat in accordance with claim 6 wherein said metallic anticipator resistor is mounted in a bore in said heatexchange means adjacent to said secured end of said bimetal strip.

9. A thermostat in accordance with claim 6 wherein said casing is separably secured to said body and includes a connecting portion bearing a pair of connector contacts connected respectively to said control contact means and said anticipator resistor, and said body includes a mating connecting portion having a pair of mating connector contacts for connection to said pair of connector contacts of said casing connecting portion.

10. A thermostat in accordance with claim 9 wherein said body further includes isolated physical projections bearing against said casing for restricing heat conduction from said body to said casing.

- 11. A thermostat in accordance with claim 6 wherein said casing includes a wall having a plurality of slots providing air passages to the exterior for promoting heat transfer between the outside air and said metallic heat-exchange means, said metallic heat-exchange means extending across the slots as a barrier against foreign materials that might enter said casing, each of said slots having a sharp bend therein for inhibiting ac cess to said metallic heat-exchange means for minimizing electric shock hazard.

12. A thennostat in accordance with claim 1 wherein said thermal relay includes an electric heating element and an operating bimetal arranged to be heated by said electric heating element for closing said relay contacts, and a compensating bimetal supporting said operating bimetal, said compensating bimetal and said operating bimetal being proportioned such that the free end of said operating bimetal is maintained at a position relatively independent of the ambient temperature at temperatures below normal room temperature.

13. A thermostat in accordance with claim 12 wherein said compensating bimetal is elongated and is supported at one end, the opposite end of said compensating bimetal being joined to said operating bimetal, and stop means disposed at the high temperature deflection side of the compensating bimetal partway along the length thereof from said supported end forreduced compensation above a predetermined temperature.

14. A thermostat in accordance with claim 1 wherein said thermal relay includes an electric heating element and an operating bimetal strip in substantially direct heat-transfer relation to said electric heating element, a compensating bimetal element attached at one end to one end of said operating bimetal strip, and a support attached to said body, said compensating bimetal element being mounted at the other erid thereof to said support such that deflection of said compensating bimetal .element is restricted by said support at temperatures above a predetermined temperature.

15. A thermal relay in accordance with claim 14 wherein said compensating bimetal element has a bend therein thereby separating it into two parts, one of said parts laying flat against said support at and above said predetermined temperature.

16. A thermostat in accordance with claim 1 wherein said supply terminal means includes first and second line terminals and said heater terminal means includes first and second load terminals, means including said relay contacts when closed for completing a circuit between said first line terminal and said first load terminal, a disconnect switch, means in said body and including said disconnect switch when closed for completing a circuit between said second line terminal and said second load terminal, and manual operating means for operating said switch and for controlling said relay contacts including a manual operator at the front of said body operable to an off position for opening said disconnect switch and for insuring the opening of said relay contacts, said manual operator being operable to an on" position for closing said disconnect switch and for releasing said relay contacts.

17. A thennostat in accordance with claim 16 wherein said relay contacts are constructed for long-life service in interrupting heater current and said disconnect switch is constructed for. carrying heater current but not for long-life service in interrupting heater current, said manual operating means having respective portions insuring the opening of said relay contacts before said disconnect switch is opened when said manual operator is operated from the on position to the off" position and for releasing said relay contacts for closing only after closing said disconnect switch when said manual operator is operated from the 011" to the off" position.

18. A thermostat in accordance with claim 16 wherein said ambient temperature-sensing means includes control contacts, said thermostat having a control circuit including said control contacts for energizing and de-energizing said thermal relay, said control circuit including a control circuit switch in series with said control contacts, said manual operating means being coupled to said control circuit switch for opening and closing the latter in coordination with the opening and closing of said disconnect switch.

19. A thermostat in accordance with claim 17 further including an anticipator resistor, said ambient temperaturesensing means including control contacts, said thermostat having a control circuit including said anticipator resistor in series with said control contacts, said control circuit including a control circuit switch in series with said anticipator resistor, said manual operating means being coupled to said control circuit switch for opening and closing the latter in coordination with the opening and closing of said disconnect switch.

20. A thermostat in accordance with claim 18 wherein said disconnect switch andsaid control circuit are commutator type switches having stationary contacts carried by said body and commutator-type contacts carried by said manual operator, said manual operating means including a cam-following member for forcing said relay contacts open and a cam on said manual operator cooperating with said cam-followin g member to cause the latter to force the relay contacts open only when said manual operating member is operated to the oft position.

21. A thennostat in accordance with claim 18 wherein said thermal relay includes an electric heating element, and wherein said ambient temperature-sensing means includes a bimetal strip and an anticipator resistor, said electric heating element, said anticipator resistor, said bimetal strip, said control contacts and said control circuit switch forming a series circuit between said first and second line terminals.

22. A thermostat including electrical contact means, an elongated ambient temperature responsive bimetal strip arranged to open and close said electrical contact means in response to changes of the bimetal strip temperature from a set point, a heat-exchange member of metal having a relatively large surface area, one end of said bimetal strip being secured to said heat-exchange member and being in direct heattransfer relation thereto, and an anticipator resistor in intimate heat-transfer relation to said heat-exchange member adjacent to said secured end of said bimetal strip, said thermostat including means connected to said anticipator resistor for causing said anticipator resistor to be energized in coordina tion with the operation of said electrical contact means.

23. A thermostat in accordance with claim 22 wherein said electrical contact means, said bimetal strip, said heatexchange member and said anticipator resistor are all contained in a thin-walled casing of electrical insulation for substantially blocking access to the contents thereof, said casing being in direct extended-area contact with said heat-exchange member.

24. A thermostat in accordance with claim 23 wherein said casing has a plurality of slots therethrough each having a sharp bend preventing straight-line access to the contents of the casing so as to minimize electrical shock hazard while providing direct exposure of at least part of the interior of the casing to the outside air.

25. A thermostat in accordance with claim 24 wherein said heat-exchange member covers all of said slots such that said casing and said heat-exchange member act together to substantially close the space within the casing against entry of foreign material.

26. A thermostat in accordance with claim 22 wherein a face of said heat-exchange member is approximately parallel with said bimetal strip and extends along essentially all of the length thereof.

27. A thermostat in accordance with claim 22 wherein said anticipator resistor is connected in series with said electrical contact means and is mounted in a bore in said heat-exchange member adjacent to the end of said bimetal strip means secured to said heat-exchange member.

said compensating bimetal element is restricted at temperatures above a predetemtined temperature.

29. A thermal relay in accordance with claim 28 wherein said compensating bimetal element has a bend therein thereby separating it into two parts, said support being flat where said compensating bimetal element is mounted, one of said parts laying flat against said support at said predetermined temperature, and said support acting as said stop means.

l l I i l 

1. A thermostat for monitoring the ambient temperature in a region to be heated and for controlling the connection of an electric supply line to an electric heater for said region, said thermostat including a body, supply terminal means for connection to an electric supply line, and heater terminal means for connection to an electric heater, a thermal relay carried by said body and having relay contacts operative to complete and to interrupt a current path between said supply terminal means and said heater terminal means, the heat of said thermal relay inducing the flow of convection air currents, and ambient temperature-sensing means differently responsive to the ambient temperature above and below a set point controlling the energization of said thermal relay, said ambient temperaturesensing means being carried by said body below said thermal relay in said convection air current.
 2. A thermostat in accordance with claim 1 further including a pair of holes on said body for enabling the mounting of said thermostat to a wall electrical box such that when it is mounted to said wall electrical box said ambient temperature-sensing means is situated below the bottom of said box.
 3. A thermostat in accordance with claim 1 wherein said ambient temperature-sensing means includes control contact means for making and breaking an energizing circuit to said thermal relay, an ambient temperature-sensing bimetal strip for controlling the opening and closing of said control contact means above and below a set point, and a closed casing of insulation containing said bimetal strip and said control contact means for guarding the contents of said casing against access and thereby minimizing electrical shock hazard.
 4. A thermostat in accordance with claim 3 wherein said casing includes a wall having a plurality of slots providing air passages to said ambient temperature-sensing means for promoting rapid response of said bimetal strip to the temperature of the convection air currents, each of said slots including a sharp bend for inhibiting access to the interior of the casing for minimizing electric shock hazard.
 5. A thermostat in accordance with claim 1 wherein said ambient temperature-sensing means is a separate unit contained in a casing secured to said body, said body and said casing having juxtaposed mechanical and electrical connecting portions and said connecting portions having formations for restricting the physical heat conduction path from said body to said separate unit.
 6. A thermostat in accordance with claim 1 wherein said ambient temperature-sensing means includes a casing of electrical insulation, control contact means in said casing for selectively energizing said thermal relay, metallic heat-exchange means disposed in said casing, a bimetal strip secured at one end thereof to said metallic heat-exchange means and arranged to operate said control contact means, and an anticipator resistor electrically connected to said control contact means and mounted in intimate heat-exchange relationship with said metallic heat-exchange means.
 7. A thermostat in accordance with claim 6 wherein said anticipator resistor is mounted adjacent to said secured end of said bimetal strip.
 8. A thermostat in accordance with claim 6 wherein said metallic anticipator resistor is mounted in a bore in said heat-exchange means adjacent to said secured end of said bimetal strip.
 9. A thermostat in accordance with claim 6 wherein said casing is separably secured to said body and includes a connecting portion bearing a pair of connector contacts connected respectively to said contrOl contact means and said anticipator resistor, and said body includes a mating connecting portion having a pair of mating connector contacts for connection to said pair of connector contacts of said casing connecting portion.
 10. A thermostat in accordance with claim 9 wherein said body further includes isolated physical projections bearing against said casing for restricing heat conduction from said body to said casing.
 11. A thermostat in accordance with claim 6 wherein said casing includes a wall having a plurality of slots providing air passages to the exterior for promoting heat transfer between the outside air and said metallic heat-exchange means, said metallic heat-exchange means extending across the slots as a barrier against foreign materials that might enter said casing, each of said slots having a sharp bend therein for inhibiting access to said metallic heat-exchange means for minimizing electric shock hazard.
 12. A thermostat in accordance with claim 1 wherein said thermal relay includes an electric heating element and an operating bimetal arranged to be heated by said electric heating element for closing said relay contacts, and a compensating bimetal supporting said operating bimetal, said compensating bimetal and said operating bimetal being proportioned such that the free end of said operating bimetal is maintained at a position relatively independent of the ambient temperature at temperatures below normal room temperature.
 13. A thermostat in accordance with claim 12 wherein said compensating bimetal is elongated and is supported at one end, the opposite end of said compensating bimetal being joined to said operating bimetal, and stop means disposed at the high temperature deflection side of the compensating bimetal partway along the length thereof from said supported end for reduced compensation above a predetermined temperature.
 14. A thermostat in accordance with claim 1 wherein said thermal relay includes an electric heating element and an operating bimetal strip in substantially direct heat-transfer relation to said electric heating element, a compensating bimetal element attached at one end to one end of said operating bimetal strip, and a support attached to said body, said compensating bimetal element being mounted at the other end thereof to said support such that deflection of said compensating bimetal element is restricted by said support at temperatures above a predetermined temperature.
 15. A thermal relay in accordance with claim 14 wherein said compensating bimetal element has a bend therein thereby separating it into two parts, one of said parts laying flat against said support at and above said predetermined temperature.
 16. A thermostat in accordance with claim 1 wherein said supply terminal means includes first and second line terminals and said heater terminal means includes first and second load terminals, means including said relay contacts when closed for completing a circuit between said first line terminal and said first load terminal, a disconnect switch, means in said body and including said disconnect switch when closed for completing a circuit between said second line terminal and said second load terminal, and manual operating means for operating said switch and for controlling said relay contacts including a manual operator at the front of said body operable to an ''''off'''' position for opening said disconnect switch and for insuring the opening of said relay contacts, said manual operator being operable to an ''''on'''' position for closing said disconnect switch and for releasing said relay contacts.
 17. A thermostat in accordance with claim 16 wherein said relay contacts are constructed for long-life service in interrupting heater current and said disconnect switch is constructed for carrying heater current but not for long-life service in interrupting heater current, said manual operating means having respective portions insuring the opening of said relay contacts before said disconnect switch is oPened when said manual operator is operated from the ''''on'''' position to the ''''off'''' position and for releasing said relay contacts for closing only after closing said disconnect switch when said manual operator is operated from the ''''on'''' to the off'''' position.
 18. A thermostat in accordance with claim 16 wherein said ambient temperature-sensing means includes control contacts, said thermostat having a control circuit including said control contacts for energizing and de-energizing said thermal relay, said control circuit including a control circuit switch in series with said control contacts, said manual operating means being coupled to said control circuit switch for opening and closing the latter in coordination with the opening and closing of said disconnect switch.
 19. A thermostat in accordance with claim 17 further including an anticipator resistor, said ambient temperature-sensing means including control contacts, said thermostat having a control circuit including said anticipator resistor in series with said control contacts, said control circuit including a control circuit switch in series with said anticipator resistor, said manual operating means being coupled to said control circuit switch for opening and closing the latter in coordination with the opening and closing of said disconnect switch.
 20. A thermostat in accordance with claim 18 wherein said disconnect switch and said control circuit are commutator-type switches having stationary contacts carried by said body and commutator-type contacts carried by said manual operator, said manual operating means including a cam-following member for forcing said relay contacts open and a cam on said manual operator cooperating with said cam-following member to cause the latter to force the relay contacts open only when said manual operating member is operated to the ''''off'''' position.
 21. A thermostat in accordance with claim 18 wherein said thermal relay includes an electric heating element, and wherein said ambient temperature-sensing means includes a bimetal strip and an anticipator resistor, said electric heating element, said anticipator resistor, said bimetal strip, said control contacts and said control circuit switch forming a series circuit between said first and second line terminals.
 22. A thermostat including electrical contact means, an elongated ambient temperature responsive bimetal strip arranged to open and close said electrical contact means in response to changes of the bimetal strip temperature from a set point, a heat-exchange member of metal having a relatively large surface area, one end of said bimetal strip being secured to said heat-exchange member and being in direct heat-transfer relation thereto, and an anticipator resistor in intimate heat-transfer relation to said heat-exchange member adjacent to said secured end of said bimetal strip, said thermostat including means connected to said anticipator resistor for causing said anticipator resistor to be energized in coordination with the operation of said electrical contact means.
 23. A thermostat in accordance with claim 22 wherein said electrical contact means, said bimetal strip, said heat-exchange member and said anticipator resistor are all contained in a thin-walled casing of electrical insulation for substantially blocking access to the contents thereof, said casing being in direct extended-area contact with said heat-exchange member.
 24. A thermostat in accordance with claim 23 wherein said casing has a plurality of slots therethrough each having a sharp bend preventing straight-line access to the contents of the casing so as to minimize electrical shock hazard while providing direct exposure of at least part of the interior of the casing to the outside air.
 25. A thermostat in accordance with claim 24 wherein said heat-exchange member covers all of said slots such that said casing and said heat-exchange member act together to substantially close the space within the casing against enTry of foreign material.
 26. A thermostat in accordance with claim 22 wherein a face of said heat-exchange member is approximately parallel with said bimetal strip and extends along essentially all of the length thereof.
 27. A thermostat in accordance with claim 22 wherein said anticipator resistor is connected in series with said electrical contact means and is mounted in a bore in said heat-exchange member adjacent to the end of said bimetal strip means secured to said heat-exchange member.
 28. A thermal relay comprising a movable contact and a companion contact engageable thereby, an operating bimetal for operating said movable contact, an electrical heating element disposed for primarily heating said operating bimetal, a compensating bimetal element attached at one end to one end of said operating bimetal, and a support, said compensating bimetal element being mounted at the other end thereof to said support, and stop means limiting deflection of at least part of said compensating bimetal element such that deflection of said compensating bimetal element is restricted at temperatures above a predetermined temperature.
 29. A thermal relay in accordance with claim 28 wherein said compensating bimetal element has a bend therein thereby separating it into two parts, said support being flat where said compensating bimetal element is mounted, one of said parts laying flat against said support at said predetermined temperature, and said support acting as said stop means. 